make is_{arch}_feature_detected! available for std

[the8472]

The feature detection macros currently can't be used in std itself due to these errors.

error: macro-expanded `macro_export` macros from the current crate cannot be referred to by absolute paths
   --> library/std/src/time.rs:30:5
    |
30  | use super::is_x86_feature_detected;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[deny(macro_expanded_macro_exports_accessed_by_absolute_paths)]` on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #52234 <https://github.com/rust-lang/rust/issues/52234>
note: the macro is defined here
   --> library/std/src/../../stdarch/crates/std_detect/src/detect/macros.rs:14:9

The workaround in this PR was suggested in this zulip discussion

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Amanieu (or someone else) soon.

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[Amanieu]

Note that with #1005, std_detect is a separate crate instead of a sub-module of std.

[the8472]

Will std be updated to use it that way in the near future?

[Amanieu]

Yes, I plan on updating the stdarch submodule in std as soon as #1052 lands, which is blocked on rust-lang/rust#82838.

[Amanieu]

Actually there might be a libs team FCP on the merge due to the const generics changes, which may take a while to resolve.

[the8472]

Well, this only for a small optimization. I'll just submit a rust PR and note that it's blocked.